This invention relates to an amplifier comprising a field effect transistor (FET).
In the manner which will later be described more in detail, an FET amplifier comprises a field effect transistor having a gate electrode and source and drain electrodes. The gate electrode is supplied with a gate bias voltage. As a driving power, a source-drain voltage is supplied between the source and the drain electrodes. The field effect transistor produces a source-drain current as an output signal of the amplifier with an output power level.
It is often desired to control the output power level in compliance with an output power control command. This is particularly desirable when the amplifier is put in operation by a battery. More specifically, the output power level is reduced in order to save the battery while the amplifier is kept in a standby state.
According to the prior art, the output power control command is supplied to a source-drain power supply circuit for supplying electric power of the source-drain voltage to the field effect transistor with the source-drain voltage controlled in compliance with the output power control command. In the meantime, the gate bias voltage is kept constant once the amplifier is put into operation.
In this manner, the gate bias voltage is kept constant even while the output power level is controlled. As a consequence, the source-drain current always flows with a certain constant current value through the field effect transistor when the amplifier is put either in class A operation or in class AB operation. The source-drain current of the constant current value flows even if the output power level is reduced. This results in an objectionably large power consumption and is disadvantageous particularly when it is desired to save the battery.
Furthermore, the field effect transistor is put in operation near its saturation point. It is possible in fact to reduce the source-drain current if the source-drain voltage is much reduced. This, however, puts the field effect transistor in operation in a non-saturated region to excessively reduce the source-drain current and to give rise to an unacceptably large distortion of the output signal.